【单选题】

1Organic compost (partially decomposed organic matter) requires four basic elements: carbon, nitrogen, air, and water. The carbon comes from dead organic matter, such as dried leaves, straw, and wood chips. The nitrogen comes from fresh or green materials, such as vegetative kitchen waste, untreated grass clippings, and animal manure. Fungi, bacteria, and other microorganisms use the carbon for energy and the nitrogen to grow and reproduce. The microorganisms secrete enzymes that break down the cells of the dead vegetation and animal matter. These enzymes are the glue that cements the soil particles into larger, coarser grains.Coarse soil crumbles easily, which aerates the soil and allows it to absorb moisture efficiently. This partially digested mixture is compost.

2 Compost is a stage of decay in which most of the organic matter has been broken down, but it may still be possible to identify individual parts such as leaves and twigs. The final phase of decay is called humus--a dark, sticky, nutrient-rich substance in which the original materials can no longer be distinguisheD、Although the terms "compost" and "humus" are often used interchangeably, they are not synonymous.
The word cements in paragraph 1 is closest in meaning toA．lifts
B．freezes
C．sorts
D．combines